*2.4. Durability Testing of Cutting Tools*

Durability (fatigue) tests were carried out with the milling of 41CrS4 structural steel (DIN 17210 standard [83]) common in mechanical engineering. It was decided to use an end mill model since wear testing involves a tremendous amount of experimentation and increased consumption of tool material. The designed and manufactured model of the tool was a holder, in which the original insert was fixed (Figure 4), and the operating conditions of which were as close as possible to the operation of a real end mill. The tests were carried out at a cutting speed *V* = 82 m/min, feed *f* = 0.15 mm/tooth, milling width *B* = 5 mm, and depth *t* = 0.5 mm. In all cases, the criterion for the cutters' failure was the achievement of wear on the flank surface of a limiting value of 350 μm. Service tool life was defined as the time it took to cut before the cutter reached the wear limit. A Stereo Discovery V12 metallographic optical microscope (ZEISS GmbH, Jena, Germany) was used to quantify wear. The curves of the wear with the cutting time were plotted based on the data obtained (the work shows averaged curves obtained based on tests of five samples of each type of tool material). A portable Accretech Handysurf profilometer (ZEISS GmbH, Jena, Germany) was used to obtain reliable data on the roughness of the processed workpiece throughout the entire test cycle. Laboratory resistance tests were carried out on a vertical milling machine with an infinitely variable feed drive VM 127M of the Russian production of JSC Votkinskiy Zavod (Votkinsk, Republic of Udmurtia, Russia).

**Figure 4.** General view of the working area of the machine during performance testing (**a**) and the design of the model of the tool for milling (**b**).
